When I enter sculpt mode half of my model goes missing?
Multires modifier is the only one which can't be repositioned in stack of modifiers, as per manual:


  The Multiresolution Modifier is the only modifier that cannot be repositioned in the stack if it means that there will be geometry or other object data created or removed before it


This means that while it can be only in the top of stack relatively to other modifiers generating geometry Sculpt mode will take into account only data generated by Multires, all others (especially generator modifiers, like Mirror) will be skipped. 



So in order to have all mesh faces available for sculpting while in Sculpt mode with Multiresolution apply Mirror modifier and edit mesh  with X-Mirror option (in Edit mode) or Toolshelf > Tools > Symmetry/Lock 
(in Sculpt mode) instead.